1. Field of Invention
The present invention relates to a two-terminal type non-linear element used as a switching element, a method of manufacturing the two-terminal type non-linear element, and a liquid crystal display panel containing the two-terminal type non-linear element.
2. Description of Related Art
In an active matrix type liquid crystal display device, the space between an active matrix substrate containing switching elements provided for respective pixel regions to form a matrix array, and an opposite substrate containing, for example, color filters, is filled with a liquid crystal, and the alignment state of the liquid crystal in each of the pixel regions is controlled to display predetermined image data. For each of the switching elements, a threeterminal element such as a thin film transistor (TFT), or a two-terminal type element such as a metal-insulator-metal (MIM) non-linear element (referred to as a xe2x80x9cMIM elementxe2x80x9d hereinafter) is generally used. The switching element having the two-terminal type element is excellent in that no cross-over short circuiting occurs, and the manufacturing process can be simplified, as compared with the three-terminal element.
In a liquid crystal display device containing the MIM elements, in order to realize a liquid crystal display panel which exhibits high image quality and high contrast in which unevenness in the display, after image phenomenon and sticking image phenomenon are not observed, it is important to satisfy the following conditions for the characteristics of the MIM element:
(1) The capacitance of the MIM element is sufficiently smaller than that of the pixel of the liquid crystal display panel;
(2) Changes in the current-voltage characteristics of the MIM element with respect to time are sufficiently small;
(3) The current-voltage characteristics of the MIM element have good symmetry;
(4) The current-voltage characteristics of the MIM element have sufficiently high steepness; and
(5) The element resistance of the MIM element is sufficiently uniform in a wide voltage range.
Namely, in order to increase the contrast, it is necessary that the capacitance of the MIM element be sufficiently small, as compared with the capacitance of the liquid crystal display panel which contains capacitances of one pixel electrode, a liquid crystal which is provided in the region driven by the pixel electrode, and a signal line provided opposite to the pixel electrode.
Also, in order to increase the contrast, the current-voltage characteristics of the MIM element should have sufficiently high steepness. In order to make the unevenness in the display unrecognizable, it is necessary for the MIM element to have sufficiently uniform resistance in a wide voltage range. In order to make the after image unrecognizable, it is necessary for the MIM element to show sufficiently small changes in the current-voltage characteristics with respect to time. Further, in order to make the sticking image unrecognizable, it is necessary for the MIM element to exhibit current-voltage characteristics having sufficiently small changes with respect to time and having good symmetry.
The xe2x80x9cafter imagexe2x80x9d is the phenomenon in which a first displayed image is observed when the display screen is switched to display a second image, and the first displayed image disappears in a short time. The xe2x80x9cimage stickingxe2x80x9d is the phenomenon in which a first image is displayed over a long period of time, the display screen is switched to display a second image, and the first image is observed for a long time. The observed first image of the latter phenomenon will be referred to as the xe2x80x9csticking imagexe2x80x9d. Further, the phrase xe2x80x9cthe current-voltage characteristics have good symmetryxe2x80x9d means that there is a sufficiently small difference between the absolute values of currents at a given voltage when a current is passed from a first conductive film to a second conductive film and when a current is passed from the second conductive film to the first conductive film.
Types of MIM elements have previously been proposed. For example, Japanese Patent Unexamined Publication No. 52-149090 proposes an MIM element containing a first conductive film of tantalum, an insulation film containing a metal oxide film formed by anodization of the first conductive film, and a second conductive film of chromium formed on the surface of the insulation film. Since the insulation film is formed by anodization of the surface of the first conductive film, the insulation film is formed with a uniform thickness without pinholes. Japanese Patent Unexamined Publication No. 57-122478 proposes that a dilute aqueous solution of citric acid is used as electrolyte for anodization. In these techniques, the characteristics (2) to (5) of the MIM element are not always sufficiently good. Namely, the changes with respect to time, the symmetry and the steepness of the current-voltage characteristics are insufficient, and the element resistance is not sufficiently uniform in a wide voltage range. Therefore, a liquid crystal display panel containing the MIM elements has a problem in that it is difficult to maintain high contrast in a wide temperature range in the panel, and it easily brings about unevenness in the display.
International Patent Application PCT/JP94/00204 (International Laid-Open No. WO94/18600) proposes a structure in which a tantalum alloy film containing tungsten is used as a first conductive film of an MIM element. In this technique, since the first conductive film of the MIM element contains not a single tantalum film but an alloy film of specific elements such as tantalum and tungsten, the characteristics (2) and (3), i.e., the changes with respect to time and symmetry of the current-voltage characteristics of the MIM element, are improved as compared with the techniques previously discussed with respect to Japanese Patent Unexamined Publication Nos. 52-149090 and 57-122478. In addition, the after image can be decreased to an unrecognizable level, and the contrast can be kept high in a wide temperature range. However, this technique does not provide sufficiently high contrast at high temperatures, and, as such, cannot be used in applications requiring high contrast at high temperatures.
An object of the present invention is to provide a two-terminal type non-linear element exhibiting the characteristics (1) to (5) required for the MIM element described above. In particular, the MIM element should exhibit steepness of the current-voltage characteristics, sufficiently small changes in the current-voltage characteristics with respect to time, and high reliability.
Another object of this invention is to provide a liquid crystal display panel containing the two-terminal type non-linear element, exhibiting high contrast and high image quality, and causing neither unevenness nor sticking image in the display.
A further object of the present invention is to provide a method of manufacturing the two-terminal type non-linear element having the above described excellent characteristics.
In accordance with the present invention, a two-terminal non-linear element (referred to as a xe2x80x9cMIM type non-linear elementxe2x80x9d hereinafter) contains a first conductive film, an insulation film and a second conductive film, which are laminated on a substrate. The insulation film is obtained by anodization of the first conductive film in a electrolyte containing a water solution. The insulation film has a relative dielectric constant of 25.5 or less, and preferably a relative dielectric constant of 24.0-25.5.
In an MIM type non-linear element of the present invention, a hydrogen spectrum of the boundary region between the first conductive film and the insulation film is obtained by elementary analysis carried out by secondary ion-mass spectrography (SIMS) using irradiation of cesium primary ions. The hydrogen spectrum in the depth direction preferably has a width of 10 nm or more, more preferably a width of 15-50 nm, at an intensity of one tenth of the peak intensity.
In accordance with the present invention, in an MIM type non-linear element, a thermal desorption spectroscopy of the first conductive film preferably has a peak temperature of the hydrogen spectrum of 300xc2x0 C. or higher, more preferably a peak temperature of 300-400xc2x0 C.
In the MIM type non-linear element of the present invention, the second conductive film is not limited to a metallic film, and includes transparent conductive films of ITO and the like.
In an MIM type non-linear element of the present invention, in particular, the capacitance of the MIM type non-linear is sufficiently small, and the steepness of the voltage-current characteristic is large. In addition, in the MIM type non-linear element of the present invention, it is possible to have extremely small changes in voltage-current characteristics over the passage of time. Also, the MIM type non-linear element of the present invention can maintain high reliability over a long period of time. In the MIM type non-linear element of the present invention, the insulation film is possibly divided into three layers including upper and lower semiconductor layers adjacent to the first and second conductive films, and an insulator layer formed between the two semiconductor layers, with the insulator layer having a band gap larger than that of the semiconductor layers. One possible reason for the changes in the current-voltage characteristics with respect to time is that the fine crystal of the insulator layer is gradually broken by the application of a voltage. In the MIM type non-linear element of the present invention, the semiconductor layer containing hydrogen and having a specified thickness is present between the insulator layer and the conductive films, particularly the first conductive film, thereby decreasing the effective voltage applied to the insulator layer. As a result, changes in the current-voltage characteristics with respect to time are possibly decreased.
In accordance with the present invention, a method of manufacturing a MIM type non-linear element contains the steps of:
(a) forming a first conductive film on a substrate;
(b) heat-treating the first conductive film at a temperature of 300xc2x0 C. or higher in an inert gas;
(c) forming an insulation film on the surface of the first conductive film by anodization of the first conductive film; and
(d) forming a second conductive film on the surface of the insulation film.
This manufacturing method is capable of obtaining the MIM type non-linear element of the present invention by carrying out a heat treatment step.
In accordance with the present invention, a liquid crystal display panel contains the MIM type non-linear element. More specifically, the liquid crystal display panel includes a first transparent substrate with signal lines arranged in a predetermined pattern on the first transparent substrate, MIM type non-linear elements connected to the signal lines, and pixel electrodes connected to the MIM type non-linear elements. The liquid crystal display panel a further includes a second substrate containing other signal lines provided opposite to the pixel electrodes, and a liquid crystal layer sealed between the first substrate and the second substrate. This liquid crystal display panel has high contrast and causes less image sticking, and thus can display high-quality images. The liquid crystal display can be used for a wide range of applications.